Tensile strength is an important attribute of electrical cables. Tensile strength may be of particular concern for power cables having long runs in vertical or substantially vertical orientations (hereinafter referred to as “vertical run”), such as in mineshafts and high-rise buildings, especially in case of large cables (having conductor sizes greater than about 53.5 mm2 or 1/0 AWG).
In this disclosure, by “run” it is meant a cable section freely standing between two consecutive bearing points.
In order to provide a sufficient safety factor, it may be necessary that the tensile strength of the electrical cable be several times the force exerted by the weight of the specific run of electrical cable. Industry-standard safety factors of up to 7 (e.g., tensile strength seven times the weight of the run of electrical cable) may be required dependent upon application.
For long vertical runs, the conductors of an electrical cable typically cannot provide sufficient tensile strength. In order to alleviate this problem, offset cable runs and/or tensile strength elements included as part of the structure of the electrical cable may be used.
In an offset cable run, a vertical run of the electrical cable may be interrupted by a bend at an angle as high as 90° or more at, for example, a junction box, and then run horizontally or substantially horizontally for some distance (typically not less than twice the diameter of the electrical cable) before resuming a vertical run. In this way, the long vertical run is split into two or more shorter vertical runs. A long vertical run may often require multiple offsets and this complicates the installation and consumes valuable real estate in a given footprint. As a result, offsets may not be practical for long vertical runs.
Tensile strength elements included as part of the structure of the electrical cable may take a number of forms.
U.S. Pat. No. 4,956,523 relates to an armored electric cable having integral tensile members to provide additional tensile strength. The tensile members are embedded in an inner polyvinyl chloride (PVC) jacket which securely grips the central insulated conductors over which it is extruded. The jacket is, in turn, securely gripped by an armor cover formed of a steel strip. Thus, in the vertical position, much of the weight of the insulated conductors, jackets, and armor coating can be supported by the tensile members without producing dangerous longitudinal slippage or creepage between them. However, with inner PVC jacket and armor cover, this cable design is very heavy.
Also, the Applicant has experienced that tensile elements provided into the interstices between insulated conductors may slip in between the conductors under tensile load at the cable operating temperatures.
U.S. Pat. No. 4,467,138 relates to a communication wire of flat construction. The cable pairs are located on opposite sides of a central reinforcing or support wire which can consist of a copper clad steel wire. Although communication wire may have long vertical runs, the structure and, especially, the weight of a communication wire is significantly different than an electrical cable for power transmission.
U.S. Pat. No. 4,002,820 relates to a power cable having an extensible ground check conductor for use in mining operations. The cable includes a cradle, at the center of which is inserted the ground check conductor. The cradle supports three helically wound power conductors made up of a plurality of strands of metallic wires covered with a layer of elastomeric insulation. The cradle is made of a semi-conducting insulating material consisting of the same elastomeric material as the insulation, but containing a predetermined amount of carbon black. The cradle also supports three grounding conductors inserted one between each power conductor. The grounding conductors are each made up of a plurality of strands of metallic wires and are covered with a semi-conducting elastomeric layer of the same material as the cradle.
German Patent Publication No. DE 32 24 597 A1 relates to a power cable containing, in the core or in the interstices of the stranded electrical conductors symmetrically distributed over the cross-section of the line, one or more optical conductors which are provided with an outer braiding or mesh made of tensile elements and which take over the entire capacity of the line. As tensile elements, steel or plastic strands or steel-copper mixed strands are considered.
“Flexible Electric Cables for Mining Applications”, page 39 (Pirelli, 2000), teaches that flexible electric cables for mining applications should not be stressed above the set-out limits for the permissible tensile forces. If higher tensile forces are to be expected, support elements have to be provided as part of the structure of the cable. A support element can be located in the center of the cable.
These problems are not limited to electrical cables with long vertical runs. Other situations may arise in which the tensile strength of electrical cables may be of particular concern.
Related art electrical cables are discussed, for example, in U.S. Patent Publication No. 2012/0082422 A1 to Sarchi et al. and in “Flexible Electric Cables for Mining Applications”, discussed above.